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Dying oaks don’t give themselves to love

A valley oak (Quercus lobata), from central coastal California, one of the 70 trees that died of natural causes studied to test whether oaks invest more in producing acorns than expected in the years prior to their demise.(Credit: Walt Koenig)

As they approach the end of their lives, a range of animals from beetles to humans have been found to put more of their resources into reproduction than expected. But until now, there as never been a test of this ‘terminal investment’ hypothesis in long-lived trees. Such a test is particularly difficult given that trees can live hundreds of years.

In a paper appearing in The American Naturalist, Walt Koenig of Cornell University and his colleagues use acorn survey data collected on over 1000 mature trees at 20 sites throughout California for periods of up to 37 years to test the terminal investment hypothesis. For a subset of the trees, the authors also used stainless steel bands wrapped around the trunks to measure annual growth.

During the course of the study, 70 of their trees died of apparently natural causes. Koenig found no evidence that dying trees increased their expected investment in either acorn production or growth during the six years prior to their demise relative to either other trees of the same species at the same site or compared to the same individual tree during even earlier years. Prior studies have found that dying trees often grow less than expected; the results presented in this paper demonstrate that such a decline in growth is not a consequence of trees ‘giving themselves to love’ just prior to their death.

A lot of folklore has been built around acorn production, which typically varies dramatically from year to year—a phenomenon known as ‘masting’ or ‘mast-fruiting’ behavior. Long-term studies such as that of Koenig’s, which has been supported by the National Science Foundation, are critical to testing the extent to which such ideas are in fact backed up by data.
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